Expanded Polystyrene (EPS) patterns are used in Evaporative Pattern Casting (EPC). The cost of EPS pattern-making through moulding cannot be justified for small batch production. As 3D printing or Additive Manufacturing (AM) is suitable for small volumes, several efforts have been made to develop these machines for EPS including Segmented Object Manufacturing (SOM) of the authors. EPS is typically processed through ablation or traditional machining, the former using hot tools in the form of profiled axisymmetric shapes, blades or wire. The efficacy of SOM machine relies on the performance of the individual sub-systems, so in the present work, the ablation studies of hot wire slicing of the machine is performed. The kerfwidth and surface roughness determine the quality of the pattern produced by ablation. The objective of the paper is to investigate the ablation process by introducing novel mathematical model to predict the kerfwidth for different power inputs and feedrates, which is further validated with experimental data. Subsequently, an empirical relationship is established to predict the surface roughness (Ra) of the sliced surface by performing regression analysis of collected experimental data. The different cutting zones have also been classified, which gives an overall understanding of slicing mechanism. The average error in surface roughness prediction is found to be around ±10%. With the help of these models, the SOM machine can produce better quality EPS patterns in terms of kerfwidth and Ra value, which finally depicts the quality of final casting produced by EPC process. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.